Changes in palaeoclimate and palaeoenvironment in the Upper Yangtze area (South China) during the Ordovician–Silurian transition

The Ordovician–Silurian transition was a critical period in geological history, during which profound changes in climatic, biotic, and oceanic conditions occurred. To explore the provenance, palaeoclimate, and palaeoredox conditions in the Sichuan Basin during the Late Ordovician–early Silurian interval, we conducted mineralogical, geochemical, and isotopic analyses of three formations (Wufeng, Guanyinqiao and Longmaxi formations) in the Xindi No. 2 well. The ternary and bivariate diagrams indicate that the provenance is mainly felsic igneous rocks and originated mainly from a collisional setting, presumably due to an active continental margin. The chemical index of alteration (CIA) values in the lower Wufeng and Longmaxi formations are relatively high (67.48–73.57), indicating a warm and humid climate. In contrast, the CIA values declined rapidly (58.30–64.66) during the late Katian to early Hirnantian, which had a fluctuating cold and dry climate and was interrupted by a transient warm and humid climate. The palaeoredox indices (Mo concentrations and Moauth/Uauth, U/Th, V/Cr, Ni/Co, and V/V + Ni values) during the Late Ordovician–early Silurian indicate two cycles of water column euxinia. The first cycle occurred during Wufeng Formation deposition, with bottom waters evolving from oxic-suboxic to suboxic-anoxic. Most samples show relatively low redox-sensitive trace element concentrations during the Guanyinqiao Formation, pointing to oxic-suboxic conditions. The second cycle, during the late Hirnantian, transitioned from oxic to euxinic water conditions. Our δ13Corg data are comparable to previously reported records and exhibit a strong correlation between the Hirnantian isotopic carbon excursion (HICE), climate change, and redox conditions. We suggest that the variations in the δ13C values are related to two elements: (1) increased photosynthetic activity under oxic water conditions, and (2) increased carbonate weathering exposed by the glacio-eustatic sea- level. In addition, the high δ13Corg values might indicate a more shelf-proximal setting during Xindi No. 2 well deposition. The δ13Corg isotopic data effectively constrain the timing of the Late Ordovician mass extinction (LOME) and the evolution of the temporal changes in the climatic and ocean redox conditions, suggesting an apparent stratigraphic coincidence between climate and redox fluctuations and two-phase extinctions, which implies a strong causal relationship. The LOME was systematically driven by the combination of cooler glacial temperatures, glacio-eustatic sea-level fluctuations, and anoxic water conditions that caused the two pulses of extinction in the Yangtze shelf sea.

The enrichment factor(EF) was applied by using aluminium as the detritus index to estimate the enrichment degree of elements in each sample, and the EF value was determined by using the formula: EF X = molar[(X/ Al) sample /(X/Al) average shale ], where X represents the element in Table 5. EF X > 1 reflects the relative enrichment of element X, while EF X < 1 indicates that element X is depleted compared to average shale 55,56 . The major element EFs of black shales and carbonaceous marls from the Wufeng-Longmaxi formations are shown in Table 5 and Fig. 5. The enrichment of CaO in the above three formations can be attributed to the existence of calcite (4.0-28%, average of 17.7%) and dolomite (2.3-27.2%,average of 14.3%) according to XRD analysis. The relatively high concentration of magnesium is also related to the dolomite content 57 . As shown in Fig. 5, the K contents of the sediments from the Wufeng, Guanyinqiao, and Longmaxi formations have relatively high K/Al ratios owing to higher illite contents 58 . Titanium, a diagenetically stable constituent of marine sediments, has an EF less than 1.0. The strong correlation between Ti and Al may also be because these two elements are dominated by detrital sources, but not autogenous enrichment 55 .
The chemical index of alteration (CIA) was applied to quantify the degree of chemical weathering 59,60 . The CIA can be calculated using molecular proportions, from the equation CIA = [Al 2 O 3 /(Al 2 O 3 + CaO * + Na 2 O + K 2 O)] × 100, where CaO * represents the concentration of CaO in silicate minerals only 61 . In this case, the CaO content must be corrected for the presence of carbonates (calcite, dolomite) and apatite. In this study, the available P 2 O 5 data was initially used for phosphate correction of CaO by the following formula: CaO * = mole CaO − mole P 2 O 5 × 10/3. If the remaining number of moles was less than that of Na 2 O, the CaO value was adopted as CaO * . Otherwise, the CaO value was assumed to be equal to Na 2 O 62 .
Rare earth elements. REE analysis and related parameters are listed in Table 3 and shown as chondrite-normalized models in Fig. 6. The samples from all three formations were normalized to chondrite values and the Eu anomaly was calculated according to McLennan 63 , i.e., Eu/Eu * = (Eu cn )/[(Sm sn × Gd cn ) 1/2 ], in which the subscript cn denotes normalization of the REE to chondrite values 63 .
The total rare earth element contents (∑REEs) from the Wufeng, Guanyinqiao and Longmaxi formations vary significantly from 40.89 to 310.23 ppm, with average values of 133.16 ppm, 111.52 ppm and 113 ppm (Table 3), which are all lower than the average value of Post-Archean Australian Shale (PAAS) (184.77 ppm) 63 . Similar Table 1. Mineral compositions of the shales and carbonaceous marls from the Wufeng-Longmaxi formations in the Xindi No. 2 well (XRD). K kaolinite, C clinochlore, I illite, S smectite, I/S illite/smectite mixed layer, C/S clinochlore/smectite mixed layer, %S mixed-layer ratio.    (Fig. 7). Most of the remaining trace elements in these three formations show slight enrichment. The trace element variation patterns in all three formations are similar (Fig. 7).
Organic carbon isotopes. Fifty-four samples were analysed for δ 13

Discussion
Provenance and tectonic setting. In general, some heavy minerals, such as zircon, are enriched gradually during sedimentary recycling 62 . According to Taylor et al. 64 , the Th/Sc ratios reflect bulk source compositions, because Sc and Th are transferred quantitatively from the source to the sediment. McLennan 63 suggests that Zr/Sc ratios can be used as tracers for zircon or heavy mineral concentrations, as zirconium is mainly concentrated in zircons, in which less resistant minerals are preferentially destroyed. Therefore, the Th/Sc and Zr/Sc ratios in this study and their bivariate plots were used to derive their sediment recycling degrees and mineral composition varieties 63,64 . The Th/Sc and Zr/Sc ratios of these three formations fluctuated from 0.69 to 4.39 (average of 1.34) and from 5.64 to 43.51 (average of 9.76, Table 4). Both of these ratios indicate a lower fractionation, and the composition is close to PAAS. These data combined with the Th/ Sc-Zr/Sc bivariate diagram ( Fig. 8) indicate that during the deposition of the Wufeng-Longmaxi formations, sedimentary recycling was minor, and few clastic components came from older sediments. Furthermore, the sedimentary recycling of the studied samples is low, indicating that these geochemical data can be used to identify the provenance.
Geochemical data of detrital sediments have stable geochemical properties during weathering, transportation, and diagenesis; therefore, they can provide reliable information on provenance 65,66 . To infer the provenance of sedimentary rocks, several authors have proposed major-based (e.g., Al 2 O 3 , TiO 2 , and K 2 O) discrimination diagrams in various studies of unknown basins 67,68 . According to Hayashi et al. 69  REE distributions in sedimentary rocks have played a vital role. The stable and unaffected properties during weathering, erosion, and early diagenesis make them valuable to provenance, leading to their special utility in tracing the source of sedimentary rocks 70,71 . The Eu anomalies in sediments are generally considered to have been inherited from the source rocks [72][73][74] . Generally, the LREE/HREE ratio of mafic rocks is low, and there is no Eu anomaly, while the LREE/HREE ratio of felsic rocks is usually high, and the Eu anomaly is significant 75,76 . The normalized abundance and pattern of chondrites indicate that all samples are characterized by LREE enrichment, HREE deficits, and distinctly negative Eu anomalies (0.46-0.92; average of 0.6). All of these results suggest that felsic source rocks are the major source rocks for the Wufeng-Longmaxi formations.
In addition, several stable elements (e.g., La, Th, Hf, Yb, Zr, REEs) have been used to deduce the provenance of sedimentary rocks due to their immobility during sedimentation [76][77][78] . On the bivariate diagram of La/Th-Hf 67 , most samples plot in or near the felsic source field (Fig. 9B). In addition, the bivariate diagram of La/Yb vs.   www.nature.com/scientificreports/ ∑REE reflects that the Wufeng-Longmaxi samples mainly plot in the granite source rocks (Fig. 9C). Overall, the provenance discrimination diagrams reveal that felsic (granitic) source rocks are the major source rocks for the studied Wufeng-Longmaxi formations.
Numerous studies have shown that the geochemical characteristics of detrital rocks are significantly controlled by the plate tectonic setting of the source area, therefore, the tectonic setting of the ancient terrains has been widely identified by using major-, trace-and rare earth element-based discrimination diagrams [79][80][81] . Bhatia and Crook 70 proposed trace element-based discrimination diagrams to differentiate four tectonic settings: continental island arc, oceanic island arc, active continental margins, and passive margins. As shown in Fig. 10, most of the studied samples fall within or adjacent to the continental island arc and active continental margin domain in diagrams of La-Th-Sc, Th-Co-Zr/10, and Th-Sc-Zr/10.
To increase the success rate of identifying the tectonic setting, Verma and Armstrong-Altrin 82 used common oxides (SiO 2 , Al 2 O 3 , Fe 2 O 3 , MgO, CaO, Na 2 O, K 2 O, TiO 2 , P 2 O 5 , and MnO) to develop two multidimensional diagrams based on the log e -ratio transformation of major oxides to discriminate arc, continental rift, and collisional settings. All the major oxides must be adjusted to 100% after excluding the loss on ignition (LOI) and regarded as (X) adj , where X represents the major oxides. These diagrams can be divided into two types based on the difference in (SiO2) adj values, low-silica type (36%-63%), and high-silica type (63-95%). More details about the calculated equations for these two types of sediments are described in Verma and Armstrong-Altrin 82 .
Recently, these diagrams have been successfully applied to discriminate the tectonic setting of older sedimentary basins [83][84][85][86] . In the present study, the (SiO 2 ) adj contents of the Wufeng-Longmaxi formations vary from 32.47 to 87.15, which can be classified as low-silica samples (n = 41) and high-silica samples (n = 13). For the high-silica diagram (Fig. 11A), eleven high-silica samples from the Wufeng and Longmaxi formations plot in the collision field, with two samples from the Guanyinqiao Formation (XD2P-B31 and XD2P-B29) plotting in the arc field. On the low-silica diagram (Fig. 11B), all forty-one low-silica samples plot in the collision field. All the discriminantfunction diagrams above reflect that the sediments of the Wufeng and Longmaxi formations mainly originated from a collisional setting, while the Guanyinqiao Formation may be derived from rift and collisional settings.

Palaeoweathering indices and palaeoclimate implications. The intensity of chemical weathering is
controlled mainly by the source rock composition, during weathering, climatic conditions, and rates of tectonic uplift of the source region (e.g., Refs. 87,88 ). During chemical weathering, alkaline metal elements such as Ca, Na, and K are largely removed from source rocks, while the concentrations of Al, Si, and Ba increase in the residue 89,90 . The degree of weathering can be quantified by using mobile and immobile element oxides such as Na 2 O, CaO, K 2 O, and Al 2 O 3 . The ternary diagram of Al 2 O 3 − (CaO * + Na 2 O) − K 2 O (A-CN-K) (molecular proportions 91 ) has been widely used to evaluate the differences in chemical composition related to chemical weathering, which can also be used to analyse the weathering history and palaeoclimate [92][93][94][95][96] . As shown in Fig. 12, the samples of the three formations merged above the plagioclase-K-feldspar join, showed a narrow linear trend and were close to the muscovite and illite fields, which are consistent with the XRD results ( Table 1). The weathering trend is parallel to the A-CN borderline and does not show any tilt to the K apex, indicating that the chemical weathering conditions are relatively stable, and excluding the effect of potassium salt metasomatism during diagenesis, which corrects CIA values for further analysis.
Among the different indices of weathering, the CIA is widely used in recent studies to quantify the degree of weathering [97][98][99] . Young and Nesbitt 100 proposed that the degree of chemical weathering is related to the climatic conditions in the source area, and is considered a method to study palaeoclimate changes. This discovery also makes CIA indicators widely used to reconstruct palaeoclimate conditions 20,[97][98][99] .
The CIA values of the Wufeng-Longmaxi formation samples were calculated according to the method of McLennan 62 , and the results are listed in Table 2. Across the Ordovician-Silurian boundary, the CIA values of black shales from the Katian stage (from D. complanatus through P. pacificus biozones) are relatively consistent (Fig. 3 These results indicate that the climatic conditions were mainly cold and dry before deposition, and the sediments experienced weak to moderate weathering, which is consistent with widespread Hirnantian glaciation [101][102][103] . However, based on the data fluctuations, there may also be short-term pulses of climate warming during this interval. In summary, during the Hirnantian stage, the climate fluctuated rather than was persistently cold and arid. A similar scenario has been reported in previous studies 97,104,105 , including approximately five glacial-interglacial cycles during the glacial period with an estimated duration of ~ 500 13 to ~ 1.0-Myr 106 . As shown in Fig. 3, our fluctuating but relatively low CIA values correlate with a pronounced δ 13 C positive excursion, which represents a match with the global δ 13 C Hirnantian excursion (HICE) 107 [109][110][111][112][113] . Among such elements, uranium (U) and molybdenum (Mo) have been extensively studied due to differences in their geochemical behaviour, and their covariation is considered to be related to specific redox conditions and processes in marine depositional systems 114 . The absorption of authigenic uranium by marine sediments starts under suboxic conditions, while the enrichment of authigenic molybdenum requires the presence of H 2 S (i.e.,  www.nature.com/scientificreports/ euxinic conditions) [115][116][117][118] . Moreover, aqueous U is completely unaffected through the particulate Mn/Fe-oxyhydroxide shuttle, but aqueous Mo may be enhanced during this process 119,120 . Based on the above differences, the autogenous U-Mo covariance mode can be used to track the redox conditions, which are determined by its EF (see definition in "Major element"). In addition, some redox indices (U/Th, V/Cr, Ni/Co, and V/V + Ni) have been widely used to derive information on the palaeo-oxygen level of depositional environments [121][122][123][124][125] . All four indices were calculated and reported as stratigraphic variations in Fig. 14.
All palaeoredox proxies indicate significant changes in redox conditions during the Late Ordovician to early Silurian, which can be identified as five distinct parts (Figs. 13, 14). For part I, during the accumulation of lower Wufeng formation (D. complantus, D. complexus, and lower part of P. Pacificus graptolite zones), all of the samples exhibit little or no enrichment for Mo and U, with (Mo/U) auth ratios ranging from 0.05 to 1.33 with a median of 0.13, which are mostly between 0.1 and 0.3 times those of seawater (Fig. 13). The U/Th, V/Cr and Ni/ Co values range from 0.16 to 0.83 (average of 0.35), 1.36-6.05 (average of 2.48) and 3.48-7.79 (average of 6.24), respectively, pointing to oxic to suboxic bottom water (Fig. 14). The above data suggest fluctuations between oxic and suboxic conditions. For part II, during the upper Wufeng Formation (upper part of the P. Pacificus graptolite zone), most samples display elevated Mo enrichment, ranging from 5.36 to 18.1 ppm, with an average of 11.19 ppm. (Mo/U) EF ratios mostly fluctuate between 0.3 and 1 times seawater, plotting closer to the anoxic end (Fig. 14). The U/Th, V/Cr, and Ni/Co values increase, varying between 0. 56 Fig. 13), suggesting suboxic bottom water conditions. The U/Th, V/Cr, and V/V + Ni ratios indicate oxic to suboxic conditions (Fig. 14). Although most of the samples point to anoxic conditions, the Ni/Co ratio still shows a trend of increasing first and decreased afterwards. Thus, careful should be taken when applying these criteria to interpret the redox conditions.
For part IV, during the lower Rhuddanian Stage (A. ascensus-C. veiculosus graptolite zones), most palaeoredox indices show similar variation patterns to that of part III, which increase first and then decrease. Most samples exhibit high average Mo concentrations of 49.67 ppm (and as large as 111 ppm), with (Mo/U) EF ratios mostly approximately 1 time that of seawater (Fig. 13), suggesting a euxinic condition. Similar conclusions were found in the interpretations of U/Th, V/Cr, Ni/Co, and V/V + Ni ratios (Fig. 14), which indicate persistent strong euxinic bottom water during the deposition of the lower part of the Longmaxi Formation.
For part V, during the middle to upper Rhuddanian Stage (upper part of A. ascensus-C. veiculosus graptolite zone), most proxies remain at a steady level compared to those in part IV. The V/Cr, Ni/Co and U/Th ratios range from 3.   www.nature.com/scientificreports/ www.nature.com/scientificreports/ (Mo/U) EF ratios mostly approximately 1 times that of seawater (Fig. 13), revealing euxinic water conditions. The above data suggest that euxinia may have been maintained during the deposition of the lower Longmaxi Formation.
Relationship of δ 13 C org variations to environmental changes and possible causes of carbon isotope excursions during the O-S interval. Our data that are derived from Xindi No. 2 well as described above clearly show water column redox variations pre-and-post Hirnantian glaciation, which can be summarized as two pulses of deepening degrees of anoxia. A parallel variation in δ 13 C org values is also well documented   , followed by subsequent redox condition variations from increasing anoxia to sudden oxic-suboxic conditions. The δ 13 C org values recorded above the HICE exhibit a − 3.2‰ negative excursion after the second pause of the Hirnantian glaciation from the upper M. persculptus zone to the lower A. ascensus zone, accompanied by rapid variations from oxic-suboxic to strong euxinic water conditions. These two abrupt shifts display strong covariations between δ 13 C org and redox conditions, indicating a significant change in the relative carbon fluxes between ocean water, organic matter, and sediments. In order to explore the spatial variation in the marine carbon isotopes during the Late Ordovician-early Silurian, we compared the lateral and vertical change in δ 13 C org values across proximal to distal areas on the Yangtze Shelf Sea (Fig. 15). Three sections were selected for geochemical analyses 20,42 , along with the Xindi No. 2 well in this study, and each section represents a lateral change from the inner-to mid-shelf. All sections are well established by graptolite biozonation, which can be used as stratigraphic correlation. As is shown in Fig. 15, the positive δ 13 C org excursions characterize the Hirnantian interval across all four sections, but by comparing the positive shift, the records appear to be diachronous. In the shallow-water setting (NBZ) recorded by Yan et al. 40 , the increase in δ 13 C org occurs in the Metabolograptus extraordinarius biozone, while this excursion can be observed in the Metabolograptus persculptus biozone in a deeper water setting from the inner-shelf (SH section) to the mid-shelf (QL section), as well in the Xindi No. 2 well. The offset of δ 13 C org values vary across all four sections, with a larger offset of ~ 3.7‰ in the Xindi No. 2 well, and of ~ 2.6‰ in the NBZ section, while relatively smaller offset values are present in the SH and QL sections (of ~ 1.2‰ and ~ 1.1‰). We suggest that this δ 13 C org gradient might be explained by the different oceanic environments between shallow-and deep-water settings. SH and QL were under anoxic-euxinic conditions during Hirnantian glaciation according to Fe speciation and Mo concentrations 20 , while the NBZ and Xindi No. 2 well have been established as oxic and oxic-suboxic during the glaciation by Zhou et al. 12 and by palaeoredox proxies in this study (Fig. 14). These spatial differences in  www.nature.com/scientificreports/ water column redox conditions were probably due to differences in depositional water depths, with shallower waters associated with more oxic conditions. Positive excursions (both δ 13 C carb and δ 13 C org ) during the Hirnantian stage have been reported from sections of the Yangtze Platform (e.g. Refs. 3,11,17,30,126,127 ) as well as from different regions worldwide(e.g. Refs. 13,14,108,128 ). Proposed geological processes that may cause positive δ 13 C excursions include the following: (1) enhanced marine productivity and organic matter burial 13,15,129,130 , (2) increased carbonate-platform weathering 128,131 , (3) dissolved inorganic carbon (DIC) with high δ 13 C 132 , and (4) sea-level eustacy and shoaling of the marine chemocline 38,127,133 . The positive δ 13 C excursion has been interpreted as the result of increased organic carbon burial, which lowered atmospheric pCO 2 , leading to a consequent 13 C enrichment 108,128,134,135 . Due to the fact that photosynthetic carbon fixation of marine phytoplankton favours 12 C relative to the carbon reservoir, the progressive increase in the burial of organic matter could have led to substantial enrichment of 13 C in seawater, leading to a positive excursion of δ 13 C (e.g. Refs. 15,128,129,136 . Based on these theories, the HICE is inferred to be indicative of a major carbon burial event during the Late Ordovician. However, this organic carbon burial causal model is yet to be reconciled with the marked lithofacies change, as the shelly limestone of the Guanyinqiao Formation is sandwiched between the pre-glaciation of the Wufeng Formation and the post-glaciation of the Longmaxi Formation black shales. Furthermore, data collected from recent studies have shown low TOC values during the Hinantian interval both on the Yangtze Platform (e.g. Refs. 30,43 ) and from global strata (e.g. Refs. 19,[137][138][139]. The carbonate weathering model proposed by Kump et al. 128 linked the δ 13 C excursion, especially that of δ 13 C carb , to the enhanced carbonate platform weathering during glacioeustatic sea-level lowstand. The strong positive δ 13 C org excursion from the Xindi No. 2 well coincident with the glacial Guanyinqiao Formation may reflect increased carbonate weathering during the Hirnantian glaciation. Although there may exhibit shortterm pulses of climate warming during the Hirnantian glaciation (Fig. 12), which could accelerate carbonate platform weathering, high-resolution studies are still required to verify this conclusion. Apart from organic carbon burial and carbonate platform weathering, other factors, including dissolved inorganic carbon (δ 13 C DIC ) and glacio-eustatic sea-level change, cannot be excluded. Dissolved inorganic carbon (DIC) with high δ 13 C values, which were possibly generated by an authigenic carbonate precipitation mechanism, may play an important but poorly understood role in the acknowledgment of the Hirnantian excursion. According to Ahm et al. 19 , glacioeustatic restriction of shallow epeiric seas may increase both lateral-and vertical gradients in δ 13 C DIC from the shallow shelf to the deep basin. The lateral gradient exhibits higher seawater δ 13 C values in shelf-proximal than in shelf-distal environments due to increased photosynthesis and carbonate weathering, which can be observed in the Canada Arctic and Baltic basins 133,140 as well in the Yangtze shelf sea 127,141 . This theory may explain such a high δ 13 C org peak (~ − 25.3‰) in the Xindi No. 2 well, as its locality is closer to the shelf-proximal unit compared to previous studies (Fig. 15). Generally, organic matter is produced by photosynthetic organisms in surface water, as a result of the uptake of 12 C, and is removed as sedimentary organic matter in deep water, causing even higher 13 C values in proximal settings 142 . A surface-to deep-water gradient has been recorded in the modern Black Sea, which is a typical example of a bottom-sulfurized and stagnant ocean, where the δ 13 C org of photosynthate in surface waters is ~ 4‰ heavier than the δ 13 C org of sedimentary organic matter in sulfidic deep waters 143 . It is worth noting that the Xindi No. 2 well was characterized by the highest δ 13 C org value among all four sections (Fig. 15), which can be explained by two factors. On the one hand, the increasing circulation of organic carbon fluxes, either by photosynthetic activity or by dissolved O 2 , contributed to the preservation of the heavier δ 13 C org under more oxic water relative to those deposited under anoxia water conditions. On the other hand, increasing carbonate weathering exposed by the glacio-eustatic sea-level fall may have further accelerated carbon input to the proximal setting, contributing to the positive δ 13 C org excursion. Thus, our high values of δ 13 C org might be Our δ 13 C org isotopic data provide a good constraint on the timing of the two-phase extinctions, combined with the aforementioned studies, which can also provide the relevant evolution of the temporal changes in climatic and ocean redox conditions (Figs. 8 and 15). From the late Katian (Late Ordovician), the Yangtze Block constantly collided with the Cathaysia Block due to Caledonian movement [146][147][148][149][150] , resulting in northwest-southeast compression that caused the constantly expanding uplifts (i.e., Chengdu, Dianqian, and Jiangnan-Xuefeng uplifts) around the Sichuan basin and a deepening of the inner Yangtze Sea [151][152][153][154][155] . During the deposition of the lower part of the Wufeng Formation, the Yangtze Sea evolved into a semi-isolated shelf sea, and gradually developed suboxic to anoxic bottom waters, which is consistent with all palaeoredox proxies (Mo-auth/U-auth, U/ Th, V/Cr, and Ni/Co ratios). The climate of the Upper Yangtze basin during this time was generally warmer and more humid, consistent with CIA values from 67.48 to 73.37 (average of 69.72).
In the early Hirnantian, glaciation started on Gondwana, and its initial phase was related to the beginning of a cool climate. With the continuous formation of ice sheets, the global sea level began to fall, and the climate became colder and drier, which is consistent with the decline in CIA values (from 69.23 to 61.51) at the bottom of the Normalograptus. extraordinarius biozone (upper part of the Wufeng Formation). As temperatures decreased and eustatic sea-level dropped, redox conditions varied from anoxic to oxic-suboxic, which is consistent with the geochemical evidence discussed in our previous chapter. The overlying Guanyinqiao Formation thus deposited carbonaceous marls with abundant and diverse shelly fauna. During the early Hirnantian period, the oceanic redox evolution of the entire Yangtze Sea was mainly caused by sea level decline, and the sea level decline itself was the result of rapid global cooling. This result highlights the close relationship between the redox evolution of the Late Ordovician water column and climate change at that time 26,40,156 . These conclusions are supported by the δ 13 C org evidence in the Upper Hirnantian samples (Fig. 15).
In the late Hirnantian stage (M. persculptus biozone), glaciation ended, marked by an abrupt eustatic sea level rise. The climate started getting warmer and moister, which is consistent with the increase in the CIA values in the upper part of the Guanyinqiao Formation (from 58.30 to 62.60). As temperatures increased and sea level rose, the water-column redox rapidly returned to euxinia, coincident with the rising ratios (Mo-auth/U-auth, U/Th, V/Cr, Ni/Co, and V/V + Ni ratios) and Mo concentrations. In the early Rudanian stage, sea level rise and widespread euxinia jointly promoted black shale deposition in the Longmaxi Formation.
These data, together with previous research from other areas in the Yangtze Sea, indicate significant climatic fluctuations, from warming to cooling at the end of the late Katian stage, and then back to a warm climate at the end of the late Hirnantian, possibly fluctuate during these two-time intervals. Correspondingly, the marine redox system changed from anoxic to oxygenated, and then suddenly evolved into euxinic bottom waters, consistent www.nature.com/scientificreports/ with two pulses of the LOME. Climatic fluctuations, coupled with oceanic anoxia, were likely responsible for the gradual end of the Late Ordovician biotic crisis.

Conclusions
This study aims to reconstruct the depositional history of the Late Ordovician-early Silurian boundary successions in the Xindi No. 2 well deposited on the Yangtze shelf sea and provides new insights into possible drivers of perturbations of the carbon isotoperecords and possible causes of the LOME. Sediments from the Wufeng-Longmaxi formations were mainly derived from the collisional settings, presumably from active continental margins based on the ternary diagrams and multidimensional diagrams. The provenance of shales and carbonaceous marls from these three formations are mainly from felsic igneous rocks and show weak sediment recycling. The CIA values of the studied samples indicate that the intensity of chemical weathering in the source area was weak to moderate. Lower Wufeng and Longmaxi formations indicate a warm and humid climate during deposition. In contrast, the CIA value suddenly began to decline in the upper part of the Wufeng Formation (Hirnantian stage) and continued to decline until the Guanyinqiao Formation, and data fluctuations during this period indicate that the cold and arid climate was interrupted by brief warm climates. The palaeoredox indices (Mo concentrations and Mo auth /U auth , U/Th, V/Cr, Ni/Co, and V/V + Ni values) during the Late Ordovician-early Silurian indicate two cycles of water column euxinia. The first cycle occurred during the accumulation of the Wufeng Formation, with bottom waters evolving from oxic-suboxic (part I) to suboxic-anoxic (part II). Most samples show relatively low concentrations of redox-sensitive trace elements during the Guanyinqiao Formation (part III), pointing to oxic-suboxic conditions. The water column transitioned from oxic to euxinic in the late Hirnantian (at the base of the Longmaxi Formation, part IV). Our δ 13 C org data are comparable to previously reported records and exhibit a strong correlation between the HICE, climate change, and redox conditions. We suggest that the variations in δ 13 C values are related to two elements: (1) increased photosynthetic activity under oxic water conditions; (2) increased carbonate weathering exposed by the glacio-eustatic sea-level. In addition, our high values of δ 13 C org compared to the other three previous sections might indicate a more shelf-proximal setting during the deposition of the Xindi No. 2 well. The δ 13 C org isotopic data effectively constrain the timing of the LOME and the evolution of the temporal changes in the climatic and ocean redox conditions, suggesting an apparent stratigraphic coincidence between climate and redox fluctuations and two-phase extinctions, which implies a strong causal relationship. The LOME was systematically driven by the combination of cooler glacial temperatures, glacio-eustatic sea-level fluctuations, and anoxic water conditions that caused the two pulses of extinction in the Yangtze shelf sea.

Data availability
All data analysed during this study are included in this published article.